Lithium ion secondary batteries, since being high in the energy density and excellent in the charge and discharge cycle characteristics, are broadly used as power sources for small-size mobile devices such as cell phones and laptop computers. Further in recent years, in consideration of the environmental problem and in growing concern for the energy saving, there have been raised demands for large-size batteries requiring a high capacity and a long life, in electric cars and hybrid electric cars, power storage fields and the like.
Lithium ion secondary batteries are generally constituted mainly of a negative electrode containing, as a negative electrode active material, a carbon material capable of occluding and releasing lithium ions, a positive electrode containing, as a positive electrode active material, a lithium composite oxide capable of occluding and releasing lithium ions, a separator separating the negative electrode and the positive electrode, and a nonaqueous electrolyte solution in which a lithium salt is dissolved in a nonaqueous solvent.
As carbon materials to be used as the negative electrode active material, amorphous carbon and graphite are used; particularly in the applications requiring a high energy density, graphite is usually used and various types of graphite-based materials are being developed.
For example, Patent Literature 1 discloses, as a negative electrode material for a lithium ion battery, a composite graphite particle having a core material composed of a graphite and a carbonaceous layer present on its surface. Patent Literature 1 describes that the carbonaceous layer is obtained by adhering an organic compound on a core material and heat-treating it at 500° C. or higher; the amount of the carbonaceous layer is 0.05 to 10 parts by mass with respect to 100 parts by mass of the core material; and the composite graphite particle has a BET specific surface area of 0.2 to 30 m2/g. Patent Literature 1 describes that the composite graphite particle has a high acceptability of lithium ions; and use thereof enables providing the lithium ion battery good in the cycle characteristics and the output and input characteristics.
Patent Literature 2 discloses that in a nonaqueous electrolyte secondary battery, a negative electrode containing mixed graphite particles of coated graphite particles coated with an amorphous carbon and non-coated graphite particles coated with no amorphous carbon is used.
Patent Literature 2 describes that the mass of the amorphous carbon in 100 parts by mass of the coated graphite particles is 0.1 to 10 parts by mass, and the specific surface area of the mixed graphite particles of the coated graphite particles and the non-coated graphite particles is 2 to 5 m2/g. Patent Literature 2 describes that use of such mixed graphite particles enables suppressing deposition of lithium during high-rate charge and suppressing the cycle deterioration due to a decrease in the internal resistance of the negative electrode.
Patent Literature 3 discloses a mixed carbon material containing a carbon material A and a carbon material B, wherein both of the carbon material A and the carbon material B are each composed of a core material composed of a graphite powder and a surface carbon substance (at least one of an amorphous carbon and a turbostratic carbon) adhering on or coating a part of its surface. Then Patent Literature 3 describes that the compression density of the carbon material A is 1.80 to 1.90 g/cm3, the compression density of the carbon material B is 1.45 to 1.65 g/cm3, and the compression density of the mixed carbon material is 1.75 to 1.84 g/cm3; and the average particle diameter of the carbon material B is 7 μm or larger and 14 μm or smaller, and is smaller than that of the carbon material A; and the specific surface area of the carbon material A is 4 m2/g or smaller, and the specific surface area of the carbon material B is 6 m2/g or smaller. Patent Literature 3 describes that a lithium ion secondary battery using such a mixed carbon material, while having a high negative electrode density and thereby having a high capacity, achieves a high charging acceptability and additionally has a small irreversible capacity.
Then, Patent Literature 4 describes that when the packing density of a negative electrode is made high by pressing in order to improve the capacity of the negative electrode, since the vicinity of the negative electrode surface is excessively compressed and the size of voids in the surface vicinity becomes smaller than that in the interior, resultantly causing a nonaqueous electrolyte solution to hardly permeate into the negative electrode, the amount of the electrolyte solution held by the negative electrode becomes insufficient, resultantly bringing about the deterioration of the charge and discharge cycle characteristics of a secondary battery. Then Patent Literature 4 describes that in order to solve such a problem, the negative electrode is fabricated by a method comprising a step of applying and thereafter drying a coating containing a fibrous carbonaceous material and a graphitic material on a current collector to thereby form an active material-containing layer having a density of 1.0 to 1.3 g/cm3, and a step of raising the density of the active material-containing layer to 1.3 to 1.6 g/cm3 by pressing. Patent Literature 4 describes that by thus fabricating the negative electrode, since the sizes of voids in the active material-containing layer of the negative electrode can be made uniform, the charge and discharge cycle life of the secondary battery can be improved.
Further Patent Literature 5 discloses a carbonaceous electrode plate for a nonaqueous electrolyte solution secondary battery composed of a carbonaceous molding sheet having a density distribution or a porosity distribution in the sheet thickness direction, and having a higher density or a lower porosity in the interior portion than in the outer surface portion. Patent Literature 5 describes that such a carbonaceous electrode plate is an excellent negative electrode in which the outer surface portion exhibits the performance as the negative electrode and simultaneously functions to cause an electrolyte solution to permeate into the electrode interior; and the interior layer plays a role of doping and de-doping more lithium and also functions as a current collector having a high conductivity.
Further Patent Literature 6 describes that a negative electrode material for a nonaqueous secondary battery contains composite graphite particles (A) made by graphitizing natural graphite and a binder and carbonaceous particles (B); the composite graphite particles (A) have a tap density of 0.5 g/cm3 or higher and 1.5 g/cm3 or lower, a BET specific surface area of 1 m2/g or higher and 10 m2/g or lower, and a particle median diameter of 5 μm or larger and 30 μm or smaller; the carbonaceous particles (B) are one or more selected from the group consisting of natural graphite, artificial graphite, resin-coated graphite and amorphous carbon; the active material density of an electrode fabricated by using such a negative electrode material is 1.8±0.03 g/cm3; and the in-electrode porosity of 7.5% or higher and 30% or lower as obtained by measuring the cross-sectional image of the electrode. Patent Literature 6 describes that use of such a negative electrode material enables providing, even in the case where the density of the active material layer is made high, a nonaqueous secondary battery having a low initial charge and discharge irreversible capacity and a high capacity, and having excellent discharge load characteristics, charging acceptability, cycle characteristics and electrolyte solution permeability.